First record of the complete mitochondrial genome of Tubifex tubifex (Müller) 1774 (Annelida; Clitellata; Oligochaeta) and phylogenetic analysis

The complete mitochondrial genome of Tubifex tubifex was analyzed using the MGISEQ-2000 platform. The size of the complete mitochondrial genome was 15,972 bp. Data pertaining to the genome, such as the presence of 13 protein-coding genes (PCGs), two rRNA genes, 22 tRNA genes, and a putative control region were submitted to NCBI (MW690579). A phylogenetic tree was constructed with the sequences of the 13 PCGs using the maximum-likelihood method. Despite only a few references available on the complete mitochondrial genome of other aquatic oligochaetes, our phylogenetic analysis revealed that the freshwater oligochaetes T. tubifex and Limnodrilus hoffmeisteri are in a cluster different from that of the earthworm group.

First record of the complete mitochondrial genome and phylogenetic analysis of Limnodrilus hoffmeisteri Claparède, 1862 (Annelida; Clitellata; Oligochaeta)

The complete mitochondrial genome of Limnodrilus hoffmeisteri was analyzed using the Illumina NovaSeq 6000 platform. The length of the complete mitochondrial genome was 15,972 bp. The sequencing data, comprising 13 protein coding genes (PCGs), 2 rRNA genes, 22 tRNA genes, and a putative control region, were submitted to the National Center for Biotechnology Information (MW732144). A phylogenetic tree was constructed based on the sequences of 13 PCGs using the maximum likelihood method. To date, only a single study has been conducted on the complete mitochondrial genome of another aquatic oligochaete. This phylogenetic tree revealed that L. hoffmeisteri is clustered with Tubifex tubifex and forms a sister group to the earthworm group.

DNA barcoding of Naididae (Annelida, Oligochaeta), based on cytochrome C oxidase gene and ITS2 region in China

Exploring the effectiveness of DNA barcoding in species identification is a prerequisite for biodiversity conservation and environmental monitoring. Aquatic oligochaetes could serve as excellent indicators in aquatic monitoring programmes. However, few studies have examined the effectiveness of DNA barcoding in these specific organisms. The mitochondrial cytochrome C oxidase (COI) gene of 83 specimens belonging to 40 species of 18 genera were sequenced in this study. The results showed that there was a barcode gap between species of Naididae and the intraspecific genetic distances of each species were smaller than interspecific genetic distances. The classification results of ABGD (Automatic Barcode Gap Discovery) were consistent with those of morphological identification, except for Tubifextubifex and Lumbriculusvariegatus. All species were successfully distinguished in the phylogenetic tree, based on the ITS2 region, which was coincident with the morphological result. Our results provided evidence that DNA barcoding can be used as an effective and convenient tool for species identification of the family Naididae and even for other aquatic oligochaetes.

Alike but different: the evolution of the Tubifex tubifex species complex (Annelida, Clitellata) through polyploidization

BACKGROUND

Tubifex tubifex is a widespread annelid characterized by considerable variability in its taxonomic characteristics and by a mixed reproductive strategy, with both parthenogenesis and biparental reproduction. In a molecular phylogenetic analysis, we detected substantial genetic variability among sympatric Tubifex spp. from the Lambro River (Milano, Italy), which we suggested comprise several cryptic species. To gain insights into the evolutionary events that generated this differentiation, we performed a cytogenetic analysis in parallel with a molecular assay. Approximately 80 cocoons of T. tubifex and T. blanchardi were collected and dissected. For each cocoon, we sequenced a fragment of the 16S rRNA from half of the sibling embryos and karyotyped the other half. To generate a robust phylogeny enabling the reconstruction of the evolutionary processes shaping the diversity of these sympatric lineages, we complemented our original 16S rRNA gene sequences with additional COI sequences.

RESULTS

The chromosome number distribution was consistent with the presence of at least six sympatric euploid chromosome complements (one diploid, one triploid, three tetraploids and one hexaploid), as confirmed by a FISH assay performed with an homologous 18S rDNA probe. All the worms with 2n = 50 chromosomes belonged to an already identified sibling species of T. tubifex, T. blanchardi. The six euploid sets were coherently arranged in the phylogeny, with each lineage grouping specimens with the same chromosome complement.

CONCLUSIONS

These results are compatible with the hypothesis that multiple polyploidization events, possibly enhanced by parthenogenesis, may have driven the evolution of the T. tubifex species complex.